1. Field
The present disclosure relates generally to fiber optics, and in particular, to optical isolators.
2. The Prior Art
3. Background
The optical isolator is an element of modern optical communication networks. Optical isolators allow light to travel in one direction, while blocking light traveling in an opposite direction. The ever-increasing speeds of today's optical networks have placed higher performance demands on optical isolators. Today, network speeds of 40 Gb/s and higher are required for many applications. Polarization Mode Dispersion (PMD), Polarization Dependent Loss (PDL), and insertion loss are important characteristics which must be minimized in any high-speed optical communication system.
FIG. 1 shows a prior art optical isolator as described in U.S. Pat. No. 4,548,478 and assigned to Fujitsu Limited of Kawasaki of Japan. The optical isolator 100 of FIG. 1 includes an optical fiber 102 from which an incident light beam 104 is launched into a first lens 106. Two birefringent plates 108 and 112 are placed on either side of a 45° Faraday rotator 110 within the path of the light beam 104. When light passes through the birefringent plate 108 in a forward direction (left to right), the angle of refraction of an ordinary-ray (o-ray) 114 and an extraordinary-ray (e-ray) 116 are different, so that a polarization separation is realized, the o-and e-rays are then directed into the Faraday rotator 110, where their planes of polarization are rotated 45°, The o- and e-rays are then directed into birefringent plate 112, which is configured to transmit the e- and o-rays in a parallel manner. These parallel beams are then focused into optical fiber 120 by second lens 118. However, light traveling in a reverse direction (from right to left) will have its e-and o-rays refracted in a different manner by the birefringent plates, causing the rays not to be focused into optical fiber 102 by lens 104.
While the optical isolator 100 of FIG. 1 performs its intended function, certain disadvantages have become evident. For example, the displacement of the e- and o-rays in space (known as walk-off) introduces insertion loss and Polarization Dependent Loss (PDL) into the isolator in the forward path. Additionally, the fact that the two beams are traveling different optical paths results in the two beams having different velocities when passing through the isolator. This results in the device not being PMD-free that may not be acceptable for modern optical communication systems.